some generation code optimizations

passed - i_pad and o_pad can be written as just string.char(0x36):rep(64) and string.char(0x5c):rep(64) respectively.
assertion error - The loop for concatenating i_key_pad and message is pointless, you could directly do i_key_pad .. message. Same for the second concat loop.
passed - left_pad could be written as just s:rep(#str - len) .. str.
passed - The loop in generate_secret could be replaced with just buf:sub(1, 20)

Thanks https://forum.minetest.net/viewtopic.php?p=429245#p429245

functions.lua

@@ -16,10 +16,8 @@ end
 -- https://en.wikipedia.org/wiki/HMAC
 local i_pad = ""
 local o_pad = ""
-for _=1,64 do
-    i_pad = i_pad .. string.char(0x36)
-    o_pad = o_pad .. string.char(0x5c)
-end
+i_pad = string.char(0x36):rep(64)
+o_pad = string.char(0x5c):rep(64)
 
 -- hmac generation
 function fediauth.hmac(key, message)
@@ -60,10 +58,7 @@ function fediauth.hmac(key, message)
 end
 
 local function left_pad(str, s, len)
-    while #str < len do
-        str = s .. str
-    end
-    return str
+    return s:rep(#str - len) .. str
 end
 
 function fediauth.generate_tfediauth(secret_b32, unix_time)
@@ -125,10 +120,7 @@ end
 
 function fediauth.generate_secret()
     local buf = minetest.sha1("" .. math.random(10000), true)
-    local s = ""
-    for i=1,20 do
-        s = s .. string.char(string.byte(buf, i))
-    end
+    local s = buf:sub(1, 20)
     return s
 end